It is the usual practice in the sealant and wet casting adhesive industry to employ polyethylene coated kraft paper as a substrate for casting both sealants and adhesives. While polyethylene coated kraft paper has good dimensional stability when it is exposed to moisture or high humidity, it has poor heat resistance, being limited to exposure to temperatures not exceeding 260.degree. F. As a consequence of the low heat tolerance of polyethylene coated kraft paper, casters, whether they are using sealants or adhesives, must operate at relatively low speeds and low temperatures to avoid melting the polyethylene layer. A further disadvantage encountered by casters, especially adhesive casters, in the use of polyethylene coated kraft paper, centers on the relatively low porosity of such a substrate. More specifically in this connection, because of the relatively low porosity of such substrates, the solvent vehicle for the adhesive, during drying, can only escape through the top of the structure, that is, through the adhesive layer. If the drying conditions are not properly controlled and there is skinning over of the adhesive before all of the solvent has been driven off, bubbles or blisters can form in the adhesive, thereby adversely affecting the bond strength of the adhesive layer. Wholly apart from the aforementioned shortcomings of polyethylene coated kraft paper as a substrate for sealant and adhesive casting, the coated paper is comparatively costly to produce both from the standpoint of material costs as well as equipment costs.